Abstract: A broad-spectrum UV photodetector with ultrahigh detectivity and rapid response speed has been achieved in β-Ga₂O₃/GaN heterojunction, in which, the β-Ga₂O₃ is synthesized by substituting oxygen for nitrogen in the top layer of the GaN matrix at high temperature. The processes and mechanism of transforming GaN with varying crystal quality into β-Ga₂O₃ at high temperatures were studied in detail. The newly formed oxide layer is a monoclinic β-Ga₂O₃ with (2¯01) preferred orientation. X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM) measurements identified oxygen vacancies and surface flatness of β-Ga₂O₃, respectively, which are closely related to the crystal quality of GaN. The oxygen vacancies and the root mean square of morphology roughness of β-Ga₂O₃ decrease with the improvement of the precursor GaN crystal quality. The cross-section transmission electron microscope (TEM) measurements showed that a hexagonal phase GaN_xO_3(1-x)/2 intermediate layer with a thickness of 5 nm exists at the interface region between β-Ga₂O₃ and hexagonal GaN. This indicates a molecular reconfiguration of the hexagonal system to a monoclinic system with oxygen substitution of nitrogen in GaN matrix. The metal-semiconductor-metal (MSM) planar structure device achieved an ultrahigh detection capability (Responsivity=2493.5 A/W, Detectivity>10¹⁶ Jones). The response time is in the order of milliseconds (τ_r=0.27 ms, τ_d1/τ_d2=0.33 ms/4.3 ms). A self-powered UV optoelectronic rapid response (τ_r=5 µs, τ_d1/τ_d2=0.13 ms/2.3 ms) with the responsivity of 0.6 mA/W and the detectivity of 5.3 × 10¹¹ Jones in the solar-blind wavelength region has been observed in the β-Ga₂O₃/GaN heterojunction without external bias. With a bias of -10 V loading, the response of the photodetector becomes a broad spectrum, covering the UVA-UVC wavelength range, and the photoresponsivity is up to 13.5 A/W. The detectivity reaches a high value of 2.6 × 10¹⁵ Jones.

Key words: β-Ga₂O₃/GaN, UV photodetector, Responsivity, Reverse substitution growth